1. Field of the Invention
The present invention relates to a zoom lens system and more particularly, though not exclusively, to an image pickup apparatus including the zoom lens system.
2. Description of the Related Art
Conventionally, with respect to zoom lens systems used in photographic cameras, camcorders, and other image pickup systems, various zoom lens systems adopt a system that performs focusing by moving a lens unit behind a first lens unit, which is adjacent to an object side (the lens unit is nearer to an image side than the first lens unit is). Such a system is referred to as a rear focusing system.
The rear focusing system has attractive features in that the relatively compact and lightweight lens unit can be moved a small amount. Additionally, because rear focusing can be performed quickly, the rear focusing system can be used for an auto focusing systems.
A rear focusing zoom lens system, including five lens units, is discussed in, for example, Japanese Patent Laid-Open Nos. 5-215967 (corresponding to U.S. Pat. No. 5,388,004), 6-317752 (corresponding to U.S. Pat. No. 5,530,592), and 2000-180724.
The zoom lens system that has five lens units discussed in Japanese Patent Laid-Open No. 5-215967 includes, in order from an object side to an image side, a first lens unit with positive refractive power, a second lens unit with negative refractive power, a third lens unit with positive refractive power, a fourth lens unit with negative refractive power, and a fifth lens unit. In this zoom lens system, magnification is mainly varied by moving the second lens unit along the optical axis, and displacements of the position of an image occurring when the magnification is varied are compensated by moving the fifth lens unit along the optical axis. In addition, at least one of the third lens unit and the fourth lens unit is moved during zooming. Focusing is performed by moving at least one of the third to fifth lens units.
The zoom lens system that has five lens units discussed in Japanese Patent Laid-Open No. 6-317752 includes, in order from an object side to an image side, a first lens unit with positive refractive power, a second lens unit with negative refractive power, a third lens unit with positive refractive power, a fourth lens unit with positive refractive power, and a fifth lens unit with positive refractive power. In this zoom lens system, during zooming from a wide-angle end to a telephoto end, the second lens unit is moved toward the image side and the third and fourth lens units are moved. Focusing is performed by moving the fourth lens unit.
The zoom lens system that has five lens units discussed in Japanese Patent Laid-Open No. 2000-180724 includes, in order from an object side to an image side, a first lens unit with positive refractive power, a second lens unit with negative refractive power, a third lens unit, a fourth lens unit with positive refractive power, and a fifth lens unit. In this zoom lens system, magnification is varied by moving the second lens unit, and displacements of the position of an image occurring when the magnification is varied are compensated by moving the third lens unit. Focusing is performed by moving the fifth lens unit.
Furthermore, another zoom lens system that has five lens units is discussed in, for example, Japanese Patent Laid-Open No. 2002-365547. This zoom lens system includes, in order from an object side to an image side, a first lens unit with positive refractive power, a second lens unit with negative refractive power, a third lens unit with positive refractive power, a fourth lens unit with negative refractive power, and a fifth lens unit with positive refractive power. In this zoom lens system, the first to fifth lens units are moved during zooming from a wide-angle end to a telephoto end.
With a reduced distance between pixels of an image pickup device, recent zoom lens systems used in optical devices (e.g., digital camera, camcorders, and personal digital assistants (PDAs)), require higher optical performance, smaller entire lens length, and a more compact size.
For a general zoom lens, increasing refractive power of each lens unit can reduce the amount of movement of each lens unit to obtain a predetermined zoom ratio, thus facilitating the reduction of the entire lens length.
However, if the refractive power of each lens unit is simply increased, aberration occurring during zooming is inevitably increased. This makes it difficult to obtain high optical performance over the entire zoom range.